1. Field of the Invention
The present invention relates to a steering coupling structure for a steering system of a motor vehicle, and more particularly to a novel steering coupling structure capable of achieving a good operating feeling of the steering system and exhibiting an excellent vibration isolating characteristic.
2. Discussion of the Prior Art
Generally, a steering system of a motor vehicle consists of a plurality of shafts and other components, for transmitting a steering effort or force (rotary power) from a steering wheel to a steering gear. In this steering system, a steering coupling structure having vibration isolating capability is interposed in the transmission line of the steering force, so as to reduce transmission of vibrations or noise received from the steering gear, road surface or suspension system, toward the steering wheel.
As one type of such a steering coupling structure, there is known one as disclosed in Publication No. 63-18837 of examined Japanese Utility Model Application, wherein an intermediate shaft for connecting a steering shaft with a steering gear box incorporates a vibration isolating rubber member. Also known is a disc type coupling structure as disclosed in Publication No. 56-35539 of examined Japanese Utility Model Application, which includes a disc-like rubber plate having mounting holes for receiving an input steering member and mounting holes for receiving an output steering member, which are alternately formed along a circle about the center axis of the rubber plate. This coupling structure further includes wires made of a suitable fibrous material, which are wound around two circumferentially adjacent ones of the above mounting holes.
The steering coupling structure as described above is required to provide sufficiently high rigidity in a torsional direction about the axis of rotation of the steering system, so as to achieve an excellent steering feeling or handling sense of the vehicle. At the same time, the coupling structure is required to exhibit a relatively soft spring characteristic in the axial direction of the steering system, so as to reduce transmission of the vibrations to the steering wheel.
However, it is extremely difficult for the known coupling structure as disclosed in the above-identified publication No. 63-18837 to have a sufficiently soft spring characteristic in the axial direction while providing sufficiently high torsional rigidity. Namely, the ratio (A/B) of the axial spring constant A(kg/mm) to the torsional rigidity B (kg.multidot.cm/deg) of this coupling structure is usually set to about 5. Thus, the known coupling structure with the intermediate shaft is incapable of achieving a good operating feeling of the steering system and excellent vibration isolating capability at the same time.
The known coupling structure as disclosed in the above-identified publication No. 56-35539 is able to provide a high degree of torsional rigidity due to the tensile strength of the wires, and a sufficiently soft spring characteristic in the axial direction due to shearing deformation of the rubber plate, assuring a good operating feeling of the steering system and excellent vibration isolating capability. In manufacturing this coupling structure, however, there are needed considerably cumbersome and complicated procedures such as winding the wires around the mounting holes and embedding the wires in the rubber plate, which eventually push up the cost of manufacture of the same.
In the known coupling structure of the above type, each of the input and output steering members of the steering system is attached to alternate ones of the mounting holes while adjacent two of the holes are surrounded by the wires. This inevitably makes the structure rather complicated and large-sized. In particular, the coupling structure of this type is difficult to be installed in recent cars which have a considerably limited space for the coupling structure owing to enlarged dwelling space and an increased number of control apparatuses, for example.